Chemical applications of topology and group theory. 30. Distortions from idealized symmetrical coordination polyhedra by lifting molecular orbital degeneracies

Abstract

Transition metal complexes with orbitally degenerate electronic states are geometrically unstable with respect to distortion from idealized symmetry to give structures with non-degenerate electronic states in accord with the Jahn-Teller theorem. This paper examines such distortions for metal complexes of the general type ML n (4 ≤ n ≤ 9) having open-shell electronic configurations. Such distortions can be of the following two types: (i) a classical Jahn-Teller distortion in which the topology of the polyhedron is maintained but the symmetry is lowered from a point group G to a smaller point group K where G is a subgroup of K ; (ii) distortions in which both the topology and symmetry of the polyhedron are changed, typically by diamond-square processes, from a point group G 1 to G 2 where G 2 is not necessarily a subgroup of G 1 . Certain octahedral metal complexes ML 6 deviating from the favored closed-shell 18-electron rare gas electronic configuration are observed to be geometrically unstable to both types of distortions, e.g., elongation of a hyperelectronic O h octahedron to a D 4 h square bipyramid in octahedral d 9 Cu(II) complexes as compared with trigonal twists of hypoelectronic early transition metal dithiolates (e.g., Mo [S 2C 2(CF 3) 2] 3) and hexamethyls (e.g., W(CH 3) 6 and Zr(CH 3) 6 2−) to give D 3 h or C 3 v trigonal prismatic complexes. Open-shell tetrahedral metal complexes may similarly be unstable to both types of distortions, e.g. elongation or compression of opposite edges of a T d tetrahedral complex ML 4 to give a D 2 d disphenoidal complex as compared with a 90° twist of a d 8 tetrahedral complex to give a D 4 h , square planar complex. Similar geometric instabilities of open-shell penta-coordinate metal complexes may be resolved by partial diamond-square distortions to intermediate points along the trigonal bipyramid/square pyramid continuum to remove all of the orbital degeneracies. Analogous diamond-square processes in open-shell superoctahedral complexes ML n ( n = 7, 8, 9) along the pentagonal bipyramid/capped octahedron, bisdisphenoid/square antiprism, and tricapped trigonal prism/capped antiprism continua, respectively, are not sufficient to remove all degeneracies. However, neither hypoelectronic nor hyperelectronic superoctahedral metal complexes are expected to be stable.